A typical ion thruster includes a thermionic electron emitter or cathode, a power source, a supply of an ionizable gas, a plasma chamber, and an ion-optic grid for electrostatically accelerating ions extracted from the plasma in a beam. A gas such as mercury vapor, argon, or xenon is metered into the plasma chamber at a relatively low pressure. Electrons emitted from the cathode are electrostatically accelerated by an anode disposed in the plasma chamber to a velocity sufficient to produce positive ions as a result of collision of the electrons with atoms of the gas. The ion beam issuing from the plasma chamber through the grid has a relatively high specific thrust that is particularly suitable for propelling a spacecraft or steering a satellite. However, an ion thruster may also be used in a ground-based laboratory vacuum chamber for ion machining, and for ion implantation of semiconductor substrates.
A common problem in conventional ion thrusters is the rapid erosion of the cathode and other internal surfaces caused by the impact of high-energy ions, in a process referred to as "sputtering." Positively charged ions created near the electron emitter are attracted to the negatively charged cathode instead of to the grid. Since the energy of the ions is greater than the sputtering threshold of the cathode material, particles are ejected from the crystal lattice of the cathode. After a relatively short period of operation, the cathode is often so eroded by sputtering that it must be replaced. While replacement of a worn ion thruster cathode in a ground-based laboratory is relatively easy, it is virtually impossible on an unmanned spacecraft; a conventional ion thruster would thus be unsuitable for use on unmanned space missions.
Several prior art patents disclose inventions that are related to the reduction or control of sputtering erosion. For example, in U.S. Pat. No. 3,452,237, a monatomic thick film of gallium is formed over the surface of a hollow tube tantalum cathode by placing the cathode in contact with a small reservoir of gallium arsenide. Apparently, there is a strong bond between the tantalum and gallium atoms that substantially increases the sputtering threshold of the coated cathode, compared to bare tantalum.
In U.S. Pat. No. 3,603,088, a shadow shield is mounted on the extreme outermost end of a hollow tantalum tube comprising the electron emitter (cathode). The shadow shield protects a heater coil surrounding the tube from sputtering damage, as well as preventing the underside coating of an adjacent alumina insulating material by sputtering material. A keeper cap in which is disposed a small aperture, is supported by the alumina insulating material at the end of the tube. The keeper cap is connected to a potential of about 300 volts, so that an arc discharge is initiated between the tantalum tube and the keeper cap, creating a plasma that reduces the negative space charge at the cathode surface. Electrons discharged from the cathode travel through the aperture in the keeper cap into an attached ion chamber.
A dual chamber plasma discharge device is disclosed in U.S. Pat. No. 4,301,391. The first chamber contains an electron emitter and a first anode. An ionizable gas is present in the first chamber at a relatively higher pressure than in the second chamber. A low voltage discharge is sustained in the first chamber, at a potential below the sputtering threshold, producing a plume of plasma that is introduced into the second (main plasma discharge) chamber. The gas pressure in the second chamber is sufficiently low that it operates as a conventional ion source, producing a higher discharge voltage plasma as electrons are accelerated toward a second, more positive anode.
After an electron enters the plasma chamber of an ion thruster, it may strike a neutral atom on its way to the anode, producing an ion and additional electrons that may collide with other atoms. To improve the probability of such a collision, it is preferable to maximize the mean path or distance between the point where the electron enters the chamber and the point where it impacts the anode. In addition, it is desirable to magnetically contain the ionized plasma that results from such collisions. Typically, a plurality of high strength magnets are placed around the periphery of the plasma chamber, defining several magnetic rings stacked above the cathode. The surface of the plasma chamber disposed intermediate the rings comprises the anode. Exemplary of this design is the ion thruster disclosed in U.S. Pat. No. 4,466,242. A further feature of the design is a movable cylindrical cathode magnet, which may be adjusted axially to produce a desired magnetic field at the cathode tip. Each ring of magnets produces a magnetic ring cusp inside the iron anode shell.
Ring magnets are also used in an ion source described in U.S. Pat. No. 4,641,031. A ferromagnetic body surrounds a cathode of the ion source, shielding the cathode from lines of magnetic flux, thereby preventing the magnetic field from obstructing electron emission.
Extraction of the ions from the plasma chamber to produce a focused beam is usually accomplished using a closely spaced perforated accelerator grid and screen grid and, preferably, a decelerator grid. The perforations in each grid must be aligned with precision, and the separation between the two (or three) grids should be minimal to provide optimum beam current. Warpage of one or more of the grids may result in a short circuit path for current to flow between their potential difference. Alignment and warpage concerns thus tend to limit the maximum practical area of conventional ion-optic grid designs.
In U.S. Pat. No. 3,744,247, a single grid plate design is disclosed that includes a layer of dielectric material interposed between a perforated metal grid plate and the plasma chamber. The dielectric material protects the plasma chamber side of the grid plate from sputtering erosion. Another embodiment uses alternating layers of the dielectric material and a metal vapor deposited as a thin film on the dielectric to increase the maximum negative potential difference that may be applied to the grid plate, relative to the positive plasma. The innermost layer of dielectric material assumes a positive potential that is almost equal to that of the plasma inside the chamber, and in effect, becomes the screen grid.
Another version of a single grid plate ion-optic system is disclosed in U.S. Pat. No. 3,697,793. The accelerator grid in that ion-optic system comprises a plurality of bars, having two metal strip electrodes separated by a dielectric glass-filled refractory material, assembled in an "egg crate" configuration. A conventional screen grid is used with the accelerator grid. Optionally, the screen grid may be eliminated and an insulating material may be applied to the bars, forming a nose on the surface of each bar that faces toward the plasma. Different voltages may be applied to the strip electrodes, on each side of the opening in the accelerator grid, to control deflection and to focus the ion beam.
While there are certain benefits that may result from using the ion-optic system of the 3,744,247 patent or that of the 3,697,793 patent, both designs suffer from serious drawbacks. In the former patent, the single grid plate does not properly focus the beam because it lacks a decelerator grid; and, the outer surface of the grid plate is subject to ion erosion. The accelerator grid of the latter patent is unduly complicated, particularly if the design is fully implemented to provide a different potential on each strip electrode.
It is therefore an object of the present invention to more effectively reduce sputtering erosion, magnetically contain the plasma and increase the mean path of electrons, and focus the ion beam, than the prior art devices just described. Advantages of the present invention in effecting each of these functions will be apparent from the attached drawings and the description of the preferred embodiments that follow.